This is a very simple example - each of the stripes is filled with ONE colour of one value - NO GRADIENT, but put next to a colour of a different value, the brighter becomes even more bright, the darker gets even more dark when it meets the other one. On the image you see gradients, that are physically not there, it's just an optical illusion. That's the rule you have to understand - you can strenghten the colour with the value contrast, and you have colour contrast with it too. Also the colour seems different alone than among other colours, so the colours affect each other. The maximum contrast you get by putting together filling colours.
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Here H. Parramon shows how it is imoprtant to get proper background colours. ON this Francesco Serra painting the background is changed. On the first one - the background is reddish, the contrast makes the olive skin and blouse of the girl more green. That shows the influence of the red background on the character. The second one has more golden background. That makes the girl faint, she dssolves into the back, it makes her less important in the picture, accenting the background. It is heavy and makes the skin colour gray. In both examples the background is also too active, drawing attention away from the girl.

Here we have the original painting, where the background is blue with some tones of green. This is the perfect choice for the background colour, because the filling colour for the yellow is blue. Here also artist made a contrast of values - where the skin is light - the background is dark, where the skin is dark - the opposite - look at the hair at the forehead, the neck, the arm. Thru the contrasts he takes the character out of the portrait into the first plane. He also accents the girl with the black outline. Notice also the presence of the filling colours in shadows on the blouse - they are green, and the blouse is faint blue in some places. The artist always thought of the filling colours while painting that.

So to sum up - with one brush stroke you don't paint just a spot, you also paint the area around it (Chevreul)

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Part 5 - Using black and white, shading

As I said before -Â the first thing that a man thinks when paints is - to make it lighter I have to add white, to make it darker - black. Well, not exactly. Let me show you.

On this quick sketch you can see shaded bucket (let's just say that you see it). Looking at a first glance it's OK. But only OK, something is missing.

Here is a bucket with good colouristics. Here greater number of colours were used, the bucket looks more 3 dimensional, the filling colour is used in the shade to liven up the picture (to have a yellow background and blue shadow is not possible, but I used it to show you how can it be done and what it does with the picture). It looks much more alive and natural. The one above looks dirty, because it was shaded just with using black and white. For a better understanding compare the highlights. So - when it's possible - don't use white.

When painting, you have to always be aware of the surrounding environment, that can affect the thing that you paint. You have to observe, if the lighter areas are affected with blue, red or yellow colour... The shadows can also have dominant red or blue, as these are the darkest colours in the light spectrum. And be aware of what you want to show, what you want to accent. Overall it is common to use more desaturated colours, using live colours just to accent something you choose to be important in the picture.
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Well, that's about it for a beginning. There are also some things to say about harmonizing the colours, tonations and rules about using them, but it's more advance. I'll write another tutorial covering those subjects, so stay tuned.